extend() now uses f_cluster instead of f_back, while map_cluster makes
sure it's not set to LONG_LAST_CLUSTER (but to the cluster before it)
when calling extend().
extend() and first_fat() now return the new cluster number or
LONG_LAST_CLUSTER, just like find_fat_free()
Hopefully I didn't break anything... Initial testing was successful.
git-svn-id: https://svn.code.sf.net/p/freedos/svn/kernel/trunk@816 6ac86273-5f31-0410-b378-82cca8765d1b
ce290a033f